The invention relates to a cathode ray tube comprising in an evacuated envelope an electron gun system for generating at least one electron beam which is focused on a target by means of at least one accelerating electron lens. Viewed in the direction of propagation of the electron beam, the lens comprises a first and a second electrode separated by a lens gap. In the second electrode an electrically conductive foil or gauze which intersects the beam is provided at a distance from the lens gap. Such cathode ray tubes are used, for example, as black-and-white or colour display tubes for television, as a television camera tube, as a projection television display tube, as an oscilloscope tube or as a tube for displaying digits or symbols. This latter type of tube is also termed a DGD tube (DGD=Data Graphic Display).
Such a cathode ray tube is known, for example, from German Patent Application No. 3,305,415 (corresponding to allowed U.S. patent application Ser. No. 458,231 filed Jan. 1, 1983) which is laid open to public inspection and which may be considered to be incorporated herein by reference. The above-mentioned Application discloses that spherical aberration can be drastically reduced by providing a curved, electrically conductive foil or gauze in the second electrode--viewed in the direction of propagation of the electron beam--of an accelerating lens of an electron gun. According to the invention described in the Patent Application the curvature of the foil or gauze must initially decrease with an increasing distance to the axis of the electron lens. The curvature preferably occurs according to a zero order Bessel function. The spherical aberration can even be made negative by providing a cylindrical collar which extends from the foil or gauze in the direction of the first electrode up to the lens gap.
In the above-described types of tube the dimensions of the spot are very important. In fact these determine the definition of the displayed or recorded television picture. There are three contributing factors which determines the spot dimensions, namely: (1) the differences in thermal emission velocities and angles of the electrons emitted from the emissive surface of the cathode, (2) the space charge of the beam, and (3) the spherical aberration of the electron lenses used. Regarding the latter factor, is that electron lenses do not focus the electron beam ideally. In general, those electrons forming the electron beam which enter an electron lens farther away from the optical axis of the lens are deflected more strongly by the lens than electrons which enter the lens closer to the axis. This is termed positive spherical aberration. The spot dimensions increase by the third power of beam parameters such as the angular aperture or the diameter of the incident electron beam. Spherical aberration is therefore sometimes termed a third order error. It was demonstrated long ago (W. Glaser, Grundlagen der Elektronenoptik, Springer Verlag, Wien 1952) that in the case of rotationally symmetrical electron lenses in which the potential beyond the optical axis is fixed by, for example, metal cylinders, a positive spherical aberration always occurs. By using curved foils, such as those following a zero order Bessel function, the spherical aberration is drastically reduced or is even made negative to compensate for the positive spherical aberration of a preceding or succeeding lens to thus reduce the spot dimensions.